So where are all the aliens? Possible answers: a) there are no aliens, b) alien civilizations self destruct, c) aliens are hiding, d) aliens are all microbes so they can’t leave home. In last week’s post on the early solar system I spent a paragraph arguing for option d, microbes. So this week I wanted to do a full post on “where are the aliens”. With so little hard data it’s reasonable to disagree on this one.
Again let’s start with the Fermi Paradox, which dates from the 1940’s. There are hundreds of billions of stars in our galaxy. There’s rapidly growing evidence that many of them have Earthlike planets. So if even a small fraction of them developed life, and an even smaller fraction developed intelligent life, there should still be millions of alien civilizations. You can see the numbers in the Drake Equation if you want. And while each alien civilization might die out quickly, if only one of those civilizations expanded at the relatively modest rate of 1000 years a hop between stars, they would populate the entire galaxy in 10 million (NOT billion) years. Robert Freitas put a more modern twist on this in a 1980 paper saying you should send out self-replicating robotic probes, not living aliens. I think the modern replicating robotic probe version is stronger since it’s where our own space exploration technology is headed. The robotic probe version also makes it clear this only has to happen once to flood the galaxy, since robots follow their program and won’t change their mind and want to go to the bathroom before they get to the next star.
Populating the galaxy as an invasive species
The most important point you want to wrap your head around here is the time to evolve aliens or humans is billions of years, but the time to flood the galaxy is millions of years. Think of aliens as an invasive species, like say Asian Carp infesting the great lakes in the US. You don’t have to assume much about what the Carp thinks, or if it’s good or evil, or whether it’s made of DNA or silicon. All you need is relentless Darwinian logic. The first Alien Carp sending out replicating robotic probes infests the entire empty galactic ecosystem. And this happens on the millions of years timescale, so second place is a billion years of evolution behind and never makes it. Game over.
The chart below covers the 13.7 billion years from the big bang to today. See the 3.5 billion of years it took to get from early Earth life to humans? Then see that thin green vertical line I put on the far right? That green line is wider than the 10 million years it takes to infest the galaxy. When you think of it this way, the odds of two alien civilizations being at the same stage of evolution is ridiculously small. A million years of evolution on Earth’s early timeline does nothing for those single celled microbes. No change. A million years of technology development takes you from throwing stones to god-like. So if humans ever meet real aliens, they’ll be either tech-gods or slime molds. This is a real drag for me as a sci-fi fan. But I don’t see any way around the numbers.
The Great Filter
Robin Hanson nicely reframed the Fermi Paradox by something he called the Great Filter in a paper from 1996. In the great filter frame, we start by acknowledging that flooding the galaxy is where humans are headed. And then realize that since aliens haven’t already flooded the galaxy, some step along our path must be highly improbable.
What’s nice about this angle is we can scientifically examine the real history of life on Earth to see if any set of steps were highly improbable. If yes, that means the great filter is behind us and we’re likely to expand into the galaxy. But if nothing unlikely has happened so far, then the great filter must lie ahead. This would be bad news. It means we are still headed toward whatever extinction event, aka great filter, caught all the other alien civilizations.
I’m obviously a big supporter of the great filter being behind us. This stance is also called the Rare Earth Hypothesis, since it implies something very rare has happened in Earth’s life history.
My favorite candidate for the great filter is multicellular life. Why? Well nearly all complex life (plants, animals, etc) is built on eukaryotic cells. But these cells got their mitochondria from an odd merger with bacteria. At a technical level this is group selection, as two completely different types of cells became merged as partners. And group selection while possible doesn’t have math on it’s side. The problem is that for any organism it’s better for cells to defect and cheat rather than sacrifice themselves and cooperate. This makes multicellular cooperation possible but mathematically unlikely. Another mathematically unlikely quirk of complex life is sexual reproduction. It’s now believed to be necessary for complex life because it’s keeps enough variation in the population to fight off parasites. Even though it’s less efficient in reproducing genes in a pure Darwinian sense. It’s a miracle multicellular life evolved and found a way to solve both the cooperation problem and the parasite problem.
So my personal take on all this is that microbes are easy, but complex life is hard. As I said in my last post “God has an inordinate fondness for microbes”.
There are plenty of valid contrary views to multicellular life being the great filter. Here’s a few:
- The Great Filter lies ahead – some innate tendency of civilizations make them self-destruct before they send out their replicating probes. Environmental destruction. War. Genocide. You name it. This is a definite possibility, but seems weak to me since we are on the brink of being able to send out replicating probes ourselves. Even if we fail, it will not be by much. And none of the current extinction candidates seems to have the deep inevitability that would apply to all types of aliens to ever have evolved.
- Lurkers – No great filter. Aliens evolved many times, but are lurking and we can’t see them. But recall that the first aliens out the gate have a billion year head start on technology. So I think the lurker idea is pretty cool, but to me it implies the very first aliens decided to become zookeepers, stopping any new young aliens (or humans) from a Darwinian expansion. So the lurker scenario is logically tied to there being an “elder race” pulling the strings out there. I would call them Thetans, but that word appears to be taken.
- Interstellar travel is super hard – Seems a bit unlikely as we are already close to having technology to do this, but ok it’s possible.
- Dude – faster than light! If you really think about it, if FTL really existed (throwing causality to the winds) then the alien problem is a billion times worse. Not only does the first robotic probe civilization flood the galaxy, they flood every galaxy in the entire universe in only a thousand years. FTL is already a joke from a physics point of view in my book, but the lack of aliens stopping by Earth on their way to work just makes FTL more crazy.
- Other Rare Earth scenarios – to me this is completely reasonable. Maybe complex intelligence with language and culture is the hard step, and if you replayed the tape of life from the dinosaurs you’d only get them again 1 in a billion tries. Or more likely it’s a combination of many very unlikely steps, none by itself prohibitive. So single celled life itself, oxygenated atmosphere, big moon, strong magnetic field to protect the atmosphere, eukaryotic cells, complex life, big brains, sociability, language. Some combo of all of the above.
- There’s more contested possibilities on wikipedia. Or invent your own. Facts are scarce and so this is a great area to make up your own opinion without wasting time reading too much.
Facts we can discover
So are there any facts we can discover in our own lifetimes that might shed some light on this? Well, the biggest is looking for life in our own solar system. If the school of thought I’m supporting is right, then there should be single celled life in our own solar system right now. Jupiter’s moon Europa or Saturn’s moon Enceladus both have water. If they have single celled life that evolved independently, that’s pretty good support for the single celled aliens theory. If they have life genetically related to Earth, then it came from meteorites transporting it around the solar system (local panspermia). This means the step of getting to single celled life may be more unlikely that it appears, which decreases the chances for single celled life in the larger galaxy. Counterintuitively, if we find complex life on Europa this would not be just a huge surprise but bad news, since it increases the chance the great filter is still ahead of us. Finally, if no other life is found in our solar system, this makes the Rare Earth case far stronger. That would be unfortunate. If all the galaxy holds are endless forms of single celled life, and maybe here and there some complex life that never made it to intelligence, then it still has poignant beauty worth exploring. But it would be terribly sad to find the galaxy is not merely quiet, but utterly dead.